Electrolytic production of high purity aluminum using ceramic inert anodes

ABSTRACT

A method of producing commercial purity aluminum in an electrolytic reduction cell comprising ceramic inert anodes is disclosed. The method produces aluminum having acceptable levels of Fe, Cu and Ni impurities. The ceramic inert anodes used in the process may comprise oxides containing Fe and Ni, as well as other oxides, metals and/or dopants.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. Ser. No.09/431,756 filed Nov. 1, 1999, now U.S. Pat. No. 6,217,739, which is acontinuation-in-part of U.S. Ser. No. 09/241,518 filed Feb. 1, 1999, nowU.S. Pat. No. 6,126,799, which is a continuation-in-part of U.S. Ser.No. 08/883,061 filed Jun. 26, 1997, now U.S. Pat. No. 5,865,980 issuedFeb. 2, 1999. This application is also a continuation-in-part of bothU.S. Ser. No. 09/542,318 filed Apr. 4, 2000 and U.S. Ser. No. 09/542,320filed Apr. 4, 2000, which are continuations-in-part of U.S. Ser. No.09/431,756 filed Nov. 1, 1999. All of these applications and patents areincorporated herein by reference.

GOVERNMENT CONTRACT

[0002] The United States Government has certain rights in this inventionpursuant to Contract No. DE-FC07-98ID13666 awarded by the United StatesDepartment of Energy.

FIELD OF THE INVENTION

[0003] The present invention relates to the electrolytic production ofaluminum. More particularly, the invention relates to the production ofcommercial purity aluminum with an electrolytic reduction cell includingceramic inert anodes.

BACKGROUND OF THE INVENTION

[0004] The energy and cost efficiency of aluminum smelting can besignificantly reduced with the use of inert, non-consumable anddimensionally stable anodes. Replacement of traditional carbon anodeswith inert anodes should allow a highly productive cell design to beutilized, thereby reducing capital costs. Significant environmentalbenefits are also possible because inert anodes produce no CO₂ or CF₄emissions. Some examples of inert anode compositions are provided inU.S. Pat. Nos. 4,374,050, 4,374,761, 4,399,008, 4,455,211, 4,582,585,4,584,172, 4,620,905, 5,794,112, 5,865,980 and 6,126,799, assigned tothe assignee of the present application. These patents are incorporatedherein by reference.

[0005] A significant challenge to the commercialization of inert anodetechnology is the anode material. Researchers have been searching forsuitable inert anode materials since the early years of the Hall-Heroultprocess. The anode material must satisfy a number of very difficultconditions. For example, the material must not react with or dissolve toany significant extent in the cryolite electrolyte. It must not reactwith oxygen or corrode in an oxygen-containing atmosphere. It should bethermally stable at temperatures of about 1,000° C. It must berelatively inexpensive and should have good mechanical strength. It musthave high electrical conductivity at the smelting cell operatingtemperatures, e.g., about 900-1,000° C., so that the voltage drop at theanode is low and stable during anode service life.

[0006] In addition to the above-noted criteria, aluminum produced withthe inert anodes should not be contaminated with constituents of theanode material to any appreciable extent. Although the use of inertanodes in aluminum electrolytic reduction cells has been proposed in thepast, the use of such inert anodes has not been put into commercialpractice. One reason for this lack of implementation has been thelong-standing inability to produce aluminum of commercial grade puritywith inert anodes. For example, impurity levels of Fe, Cu and/or Ni havebeen found to be unacceptably high in aluminum produced with known inertanode materials.

[0007] The present invention has been developed in view of theforegoing, and to address other deficiencies of the prior art.

SUMMARY OF THE INVENTION

[0008] An aspect of the present invention is to provide a process forproducing high purity aluminum using inert anodes. The method includesthe steps of passing current between a ceramic inert anode and a cathodethrough a bath comprising an electrolyte and aluminum oxide, andrecovering aluminum comprising a maximum of 0.2 weight percent Fe, 0.1weight percent Cu, and 0.034 weight percent Ni.

[0009] Another aspect of the present invention is to provide a method ofmaking a ceramic inert anode that is useful for producing commercialpurity aluminum. The method includes the step of mixing metal oxidepowders, and sintering the metal oxide powder mixture in a substantiallyinert atmosphere. A preferred atmosphere comprises argon and from 5 to5,000 ppm oxygen.

[0010] Additional aspects and advantages of the invention will occur topersons skilled in the art from the following detailed descriptionthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a partially schematic sectional view of an electrolyticcell with an inert anode that is used to produce commercial purityaluminum in accordance with the present invention.

[0012]FIG. 2 is a ternary phase diagram illustrating amounts of iron,nickel and zinc oxides present in a ceramic inert anode that may be usedto make commercial purity aluminum in accordance with an embodiment ofthe present invention.

[0013]FIG. 3 is a ternary phase diagram illustrating amounts of iron,nickel and cobalt oxides present in a ceramic inert anode that may beused to make commercial purity aluminum in accordance with anotherembodiment of the present invention.

[0014]FIG. 4 is a graph illustrating Fe, Cu and Ni impurity levels ofaluminum produced during a 90 hour test with an Fe—Ni—Zn oxide ceramicinert anode of the present invention.

[0015]FIG. 5 is a graph illustrating electrical conductivity versustemperature of an Fe—Ni—Zn oxide ceramic inert anode material of thepresent invention.

DETAILED DESCRIPTION

[0016]FIG. 1 schematically illustrates an electrolytic cell for theproduction of commercial purity aluminum which includes a ceramic inertanode in accordance with an embodiment of the present invention. Thecell includes an inner crucible 10 inside a protection crucible 20. Acryolite bath 30 is contained in the inner crucible 10, and a cathode 40is provided in the bath 30. A ceramic inert anode 50 is positioned inthe bath 30. An alumina feed tube 60 extends partially into the innercrucible 10 above the bath 30. The cathode 40 and ceramic inert anode 50are separated by a distance 70 known as the anode-cathode distance(ACD). Commercial purity aluminum 80 produced during a run is depositedon the cathode 40 and on the bottom of the crucible 10.

[0017] As used herein, the term “ceramic inert anode” means asubstantially nonconsumable, ceramic-containing anode which possessessatisfactory corrosion resistance and stability during the aluminumproduction process. The ceramic inert anode may comprise oxides such asiron and nickel oxides plus optional additives and/or dopants.

[0018] As used herein, the term “commercial purity aluminum” meansaluminum which meets commercial purity standards upon production by anelectrolytic reduction process. The commercial purity aluminum comprisesa maximum of 0.2 weight percent Fe, 0.1 weight percent Cu, and 0.034weight percent Ni. In a preferred embodiment, the commercial purityaluminum comprises a maximum of 0.15 weight percent Fe, 0.034 weightpercent Cu, and 0.03 weight percent Ni. More preferably, the commercialpurity aluminum comprises a maximum of 0.13 weight percent Fe, 0.03weight percent Cu, and 0.03 weight percent Ni. Preferably, thecommercial purity aluminum also meets the following weight percentagestandards for other types of impurities: 0.2 maximum Si, 0.03 Zn. and0.03 Co. The Si impurity level is more preferably kept below 0.15 or0.10 weight percent. It is noted that for every numerical range or limitset forth herein, all numbers with the range or limit including everyfraction or decimal between its stated minimum and maximum, areconsidered to be designated and disclosed by this description.

[0019] At least a portion of the inert anode of the present inventionpreferably comprises at least about 90 weight percent ceramic, forexample, at least about 95 weight percent. In a particular embodiment,at least a portion of the inert anode is made entirely of a ceramicmaterial. The inert anode may optionally include additives and/ordopants in amounts up to about 10 weight percent, for example, fromabout 0.1 to about 5 weight percent. Suitable additives include metalssuch as Cu, Ag, Pd, Pt and the like, e.g., in amounts of from about 0.1to about 8 weight percent of the ceramic inert anode. Suitable dopantsinclude oxides of Co, Cr, Al, Ga, Ge, Hf, In, Ir, Mo, Mn, Nb, Os, Re,Rh, Ru, Se, Si, Sn, Ti, V, W, Zr, Li, Ca, Ce, Y and F. Preferred dopantsinclude oxides of Al, Mn, Nb, Ti, V, Zr and F. The dopants may be used,for example, to increase the electrical conductivity of the ceramicinert anode. It is desirable to stabilize electrical conductivity in theHall cell operating environment. This can be achieved by the addition ofsuitable dopants and/or additives.

[0020] The ceramic preferably comprises iron and nickel oxides, and atleast one additional oxide such as zinc oxide and/or cobalt oxide. Forexample, the ceramic may be of the formula: Ni_(1-x-y)Fe_(2-x)MyO; whereM is preferably Zn and/or Co; x is from 0 to 0.5; and y is from 0 to0.6. More preferably X is from 0.05 to 0.2, and y is from 0.01 to 0.5.

[0021] Table 1 lists some ternary Fe—Ni—Zn—O materials that may besuitable for use as the ceramic an inert anode. TABLE 1 Sam- ple NominalElemental wt. % Structural I.D. Composition Fe, Ni, Zn Types 5412NiFe₂O₄ 48, 23.0, 0.15 NiFe₂O₄ 5324 NiFe₂O₄ + NiO 34, 36, 0.06 NiFe₂O₄,NiO E4 Zn_(0.05)Ni_(0.95)Fe₂O₄ 43, 22, 1.4 NiFe₂O₄, TU* E3Zn_(0.1)Ni_(0.9)Fe₂O₄ 43, 20, 2.7 NiFe₂O₄, TU* E2Zn_(0.25)Ni_(0.75)Fe₂O₄ 40, 15, 5.9 NiFe₂O₄, TU* E1Zn_(0.25)Ni_(0.75)Fe_(1.90)O₄ 45, 18, 7.8 NiFe₂O₄, TU* EZn_(0.5)Ni_(0.5)Fe₂O₄ 45, 12, 13 (ZnNi)Fe₂O₄, TP⁺ZNO^(S) F ZnFe₂O₄ 43,0.03, 24 ZnFe₂O₄, TP⁺ZnO H Zn_(0.5)NiFe_(1.5)O₄ 33, 23, 13 (ZnNi)Fe₂O₄,NiO^(S) J Zn_(0.5)Ni_(1.5)FeO₄ 26, 39, 10 NiFe₂O₄, MP⁺NiO L ZnNiFeO₄ 22,23, 27 (ZnNi)Fe₂O₄, NiO^(S), ZnO ZD6 Zn_(0.05)Ni_(1.05)Fe_(1.9)O₄ 40,24, 1.3 NiFe₂O₄, TU* ZD5 Zn_(0.1)Ni_(1.1)Fe_(1.8)O₄ 29, 18, 2.3 NiFe₂O₄,TU* ZD3 Zn_(0.12)Ni_(0.94)Fe_(1.88)O₄ 43, 23, 3.2 NiFe₂O₄, TU* ZD1Zn_(0.12)Ni_(0.94)Fe_(1.88)O₄ 40, 20, 11 (ZnNi)Fe₂O₄, TU* DHZn_(0.18)Ni_(0.96)Fe_(1.8)O₄ 42, 23, 4.9 NiFe₂O₄, TP⁺NiO DIZn_(0.08)Ni_(1.17)Fe_(1.5)O₄ 38, 30, 2.4 NiFe₂O₄, MP⁺NiO, TU* DJZn_(0.17)Ni_(1.1)Fe_(1.5)O₄ 36, 29, 4.8 NiFe₂O₄, MP⁺NiO BC2Zn_(0.33)Ni_(0.67)O 0.11, 52, 25 NiO^(S), TU*

[0022]FIG. 2 is a ternary phase diagram illustrating the amounts ofFe₂O₃, NiO and ZnO starting materials used to make the compositionslisted in Table 1, which may be used as the ceramic of the inert anodes.Such ceramic inert anodes may in turn be used to produce commercialpurity aluminum in accordance with the present invention.

[0023] In one embodiment, when Fe₂O₃, NiO and ZnO are used as startingmaterials for making an inert anode, they are typically mixed togetherin ratios of 20 to 99.09 mole percent NiO, 0.01 to 51 mole percentFe₂O₃, and zero to 30 mole percent ZnO. Perferably, such startingmaterials are mixed together in ratios of 45 to 65 mole percent NiO, 20to 45 mole percent Fe₂O₃, and 0.01 to 22 mole percent ZnO.

[0024] Table 2 lists some ternary Fe₂O₃/NiO/CoO materials that may besuitable as the ceramic of an inert anode. TABLE 2 Analyzed ElementalNominal wt. % Structural Sample I.D. Composition Fe, Ni, Co Types CFCoFe₂O₄ 44, 0.17, 24 CoFe₂O₄ NCF1 Ni_(0.5)Co_(0.5)Fe₂O₄ 44, 12, 11NiFe₂O₄ NCF2 Ni_(0.7)Co_(0.3)Fe₂O₄ 45, 16, 7.6 NiFe₂O₄ NCF3Ni_(0.7)Co_(0.3)Fe_(1.95)O₄ 42, 18, 6.9 NiFe₂O₄, TU* NCF4Ni_(0.85)Co_(0.15)Fe_(1.95)O₄ 44, 20, 3.4 NiFe₂O₄ NCF5Ni_(0.85)Co_(0.5)Fe_(1.9)O₄ 45, 20, 7.0 NiFe₂O₄, NiO, TU* NF NiFe₂O₄ 48,23, 0 N/A

[0025]FIG. 3 is a ternary phase diagram illustrating the amounts ofFe₂O₃, NiO and CoO starting materials used to make the compositionslisted in Table 2, which may be used as the ceramic of the inert anodes.Such ceramic inert anodes may in turn be used to produce commercialpurity aluminum in accordance with the present invention

[0026] The inert anodes may be formed by techniques such as powdersintering, sol-gel processes, slip casting and spray forming.Preferably, the inert anodes are formed by powder techniques in whichpowders comprising the oxides and any dopants are pressed and sintered.The inert anode may comprise a monolithic component of such materials,or may comprise a substrate having at least one coating or layer of suchmaterial.

[0027] The ceramic powders, such as NiO, Fe₂O₃ and ZnO or CoO, may beblended in a mixer. Optionally, the blended ceramic powders may beground to a smaller size before being transferred to a furnace wherethey are calcined, e.g., for 12 hours at 1,250° C. The calcinationproduces a mixture made from oxide phases, for example, as illustratedin FIGS. 2 and 3. If desired, the mixture may include other oxidepowders such as Cr₂O₃ and/or other dopants.

[0028] The oxide mixture may be sent to a ball mill where it is groundto an average particle size of approximately 10 microns. The fine oxideparticles are blended with a polymeric binder and water to make a slurryin a spray dryer. About 1-10 parts by weight of an organic polymericbinder may be added to 100 parts by weight of the oxide particles. Somesuitable binders include polyvinyl alcohol, acrylic polymers,polyglycols, polyvinyl acetate, polyisobutylene, polycarbonates,polystyrene, polyacrylates, and mixtures and copolymers thereof.Preferably, about 3-6 parts by weight of the binder are added to 100parts by weight of the oxides. The slurry contains, e.g., about 60weight percent solids and about 40 weight percent water. Spray dryingthe slurry produces dry agglomerates of the oxides.

[0029] The spray dried oxide material may be sent to a press where it isisostatically pressed, for example at 10,000 to 40,000 psi, into anodeshapes. A pressure of about 20,000 psi is particularly suitable for manyapplications. The pressed shapes may be sintered in a controlledatmosphere furnace supplied with, for example, argon/oxygen,nitrogen/oxygen, H₂/H₂O or Co/Co₂ gas mixtures, as well as nitrogen, airor oxygen atmospheres. For example, the gas supplied during sinteringmay contain about 5-5,000 ppm oxygen, e.g., about 100 ppm, while theremainder of the gaseous atmosphere may comprise an inert gas such asnitrogen or argon. Sintering temperatures of 1,000-1,400° C. may besuitable. The furnace is typically operated at about 1,250-1,295° C. for2-4 hours. The sintering process bums out any polymeric binder from theanode shapes.

[0030] The sintered anode may be connected to a suitable electricallyconductive support member within an electrolytic metal production cellby means such as welding, brazing, mechanically fastening, cementing andthe like.

[0031] The inert anode may include a ceramic as described abovesuccessively connected in series to a cermet transition region and anickel end. A nickel or nickel-chromium alloy rod may be welded to thenickel end. The cermet transition region, for example, may include fourlayers of graded composition, ranging from 25 weight percent Ni adjacentthe ceramic end and then 50, 75 and 100 weight percent Ni, balance theoxide powders described above.

[0032] We prepared an inert anode composition of 65.65 weight percentFe₂O₃, 32.35 weight percent NiO and 2 weight percent ZnO in accordancewith the procedures described above having a diameter of about ⅝ inchand a length of about 5 inches. The starting oxides were ground,calcined and spray dried, followed by isostatic pressing at 20,000 psiand sintering at 1,295° C. in an atmosphere of nitrogen and 100 ppmoxygen. The composition was evaluated in a Hall-Heroult test cellsimilar to that schematically illustrated in FIG. 1. The cell wasoperated for 90 hours at 960° C., with an aluminum fluoride to sodiumfluoride bath ratio of 1.1 and alumina concentration maintained nearsaturation at about 7-7.5 weight percent. The impurity concentrations inaluminum produced by the cell are shown in Table 3. The impurity valuesshown in Table 3 were taken at different times up to 90 hours. TABLE 3Time (hours) Fe Cu Ni 0 0.057 0.003 0.002 1 0.056 0.003 0.002 23 0.0790.005 0.009 47 0.110 0.006 0.021 72 0.100 0.006 0.027 90 0.133 0.0060.031

[0033] The results are graphically shown in FIG. 4. The results in Table3 and FIG. 4 show low levels of aluminum contamination by the ceramicinert anode. In addition, the inert anode wear rate was extremely low.Optimization of processing parameters and cell operation may furtherimprove the purity of aluminum produced in accordance with theinvention.

[0034]FIG. 5 is a graph illustrating electrical conductivity of anFe—Ni—Zn oxide inert anode material at different temperatures. Theceramic inert anode material was except it was sintered in an atmosphereof argon with about 100 ppm oxygen. Electrical conductivity was measuredby a four-probe DC technique in argon as a function temperature rangingfrom room temperature to 1,000° C. At each temperature, the voltage andcurrent was measured, and the electrical conductivity was obtained byOhm's law. As shown in FIG. 5, at temperatures of about 900 to 1,000° C.typical of operating aluminum production cells, the electricalconductivity of the ceramic inert anode material is greater than 30S/cm, and may reach 40 S/cm or higher at such temperatures. In additionto high electrical conductivity, the ceramic inert anode exhibited goodstability characteristics. During a three-week test at 960° C., theanode maintained about 75% of its initial conductivity.

[0035] The present ceramic inert anodes are particularly useful inelectrolytic cells for aluminum production operated at temperatures inthe range of about 800-1,000° C. A particularly preferred cell operatesat a temperature of about 900-980° C., preferably about 930-970° C. Anelectric current is passed between the inert anode and a cathode througha molten salt bath comprising an electrolyte and an oxide of the metalto be collected. In a preferred cell for aluminum production, theelectrolyte comprises aluminum fluoride and sodium fluoride and themetal oxide is alumina. The weight ratio of sodium fluoride to aluminumfluoride is about 0.7 to 1.25, preferably about 1.0 to 1.20. Theelectrolyte may also contain calcium fluoride, lithium fluoride and/ormagnesium fluoride.

[0036] While the invention has been described in terms of preferredembodiments, various changes, additions and modifications may be madewithout departing from the scope of the invention as set forth in thefollowing claims.

What is claimed is:
 1. A method of producing commercial purity aluminumcomprising: passing current between a ceramic inert anode and a cathodethrough a bath comprising an electrolyte and aluminum oxide; andrecovering aluminum comprising a maximum of 0.2 weight percent Fe, amaximum of 0.1 weight percent Cu, and a maximum of 0.034 weight percentNi.
 2. The method of claim 1, wherein the ceramic inert anode comprisesan oxide containing Fe.
 3. The method of claim 1, wherein the ceramicinert anode comprises an oxide containing Ni.
 4. The method of claim 1,wherein the ceramic inert anode comprises an oxide containing Fe and Ni.5. The method of claim 4, wherein the ceramic inert anode furthercomprises Zn oxide and/or Co oxide.
 6. The method of claim 1, whereinthe ceramic inert anode is made from Fe₂O₃, NiO and ZnO.
 7. The methodof claim 1, wherein the ceramic inert anode comprises at least oneceramic phase of the formula Ni_(1-x-y)Fe_(2-x)MyO₄, where M is Znand/or Co, x is from 0 to 0.5 and y is from 0 to 0.6.
 8. The method ofclaim 7, wherein M is Zn.
 9. The method of claim 8, wherein x is from0.05 to 0.2 and y is from 0.01 to 0.5.
 10. The method of claim 7,wherein M is Co.
 11. The method of claim 10, wherein x is from 0.05 to0.2 and y is from 0.01 to 0.5.
 12. The method of claim 1, wherein theceramic inert anode is made from a composition comprising about 65.65weight percent Fe₂O₃, about 32.35 weight percent NiO, and about 2 weightpercent ZnO.
 13. The method of claim 1, wherein the ceramic inert anodecomprises at least one metal in a total amount of up to 10 weightpercent.
 14. The method of claim 13, wherein the at least one metalcomprises Cu, Ag, Pd, Pt or a combination thereof.
 15. The method ofclaim 14, wherein the at least one metal comprises from bout 0.1 toabout 8 weight percent of the ceramic inert anode.
 16. The method ofclaim 1, wherein the ceramic inert anode further comprises at least onedopant selected from oxides of Co, Cr, Al, Ga, Ge, Hf, In, Ir, Mo, Mn,Nb, Os, Re, Rh, Ru, Se, Si, Sn, Ti, V, W, Zr, Li, Ca, Ce, Y and F in atotal amount of up to 10 weight percent.
 17. The method of claim 16,wherein the at least one dopant is selected from oxides of Al, Mn, Nb,Ti, V, Zr and F.
 18. The method of claim 1, wherein the ceramic inertanode has an electrical conductivity of at least about 30 S/cm at atemperature of 1,000° C.
 19. The method of claim 1, wherein the ceramicinert anode has an electrical conductivity of at least about 40 S/cm ata temperature of 1,000° C.
 20. The method of claim 1, wherein therecovered aluminum comprises less than 0.18 weight percent Fe.
 21. Themethod of claim 1, wherein the recovered aluminum comprises a maximum of0.15 weight percent Fe, 0.034 weight percent Cu, and 0.03 weight percentNi.
 22. The method of claim 1, wherein the recovered aluminum comprisesa maximum of 0.13 weight percent Fe, 0.03 weight percent Cu, and 0.03weight percent Ni.
 23. The method of claim 1, wherein the recoveredaluminum further comprises a maximum of 0.2 weight percent Si, 0.03weight percent Zn, and 0.03 weight percent Co.
 24. The method of claim1, wherein the recovered aluminum comprises a maximum of 0.10 weightpercent of the total of the Cu, Ni and Co.
 25. A method of making aceramic inert anode for producing commercial purity aluminum, the methodcomprising: mixing metal oxide powders; and sintering the metal oxidepowder mixture in a substantially inert atmosphere.
 26. The method ofclaim 25, wherein the substantially inert atmosphere comprises argon.27. The method of claim 26, wherein the substantially inert atmospherecomprises oxygen.
 28. The method of claim 27, wherein the oxygencomprises from about 5 to about 5,000 ppm of the substantially inertatmosphere.
 29. The method of claim 27 wherein the oxygen comprises fromabout 50 to about 500 ppm of the substantially inert atmosphere.